Evolution of insect olfactory genes

The sense of smell is one of the oldest senses, dating all the way back to the first animals 600 million years ago. The power to detect and discriminate odor molecules is often strongly connected to survival and reproductive success. This strong selective pressure has led to highly sensitive olfactory systems across animal phyla. The olfactory systems of vertebrates, mollusks, arthropods and nematodes share many features that are intriguingly similar. Although there is an evolutionary convergence towards a conserved organization of signaling pathways in vertebrate and invertebrate olfactory systems the gene families involved in odorant transfer to the receptors and the receptors themselves were recruited to olfactory systems independently in different animal lineages. In insects mainly two receptor families have been described as detectors of olfactory cues: Olfactory receptors (OR) and ionotropic receptors (IR). While IRs are expressed in chemosensory tissue across protostomes, ORs were only found in insects. A similar situation was described for odorant binding proteins (OBP). Both gene families are thought to have evolved after the insect ancestor colonized land, causing a need to detect airborne chemicals. Since all previous assumptions referred to studies on flying insects, the major aim of my thesis was to investigate the olfactory system of the two primarily wingless insects, Lepismachilis y-signata (Archaeognatha: Machilidae) and Thermobia domestica (Zygentoma, Lepismatidae). The results of this thesis suggest that insect ORs appeared after insects´ conquered land, maybe as adaptation to the insect flight and the demands on sensory speed and acuity rose thereby. The first terrestrial insects might have used the evolutionary older IRs for odor detection as the closest relatives of hexapods, the crustaceans do. OBPs evolved earlier and independently from ORs. After the evolution of ORs, OBPs might be coopted secondarily into OR/Orco based olfactory pathways.